U.S. Pat. Nos. 4,859,858 and 4,859,859, both entitled "GAS ANALYZER", were issued to Knodle et al on Aug. 22, 1989. Both patents disclose apparatus for outputting a signal indicative of the concentration of a designated gas in a sample being monitored by the apparatus. Apparatus of that character is also disclosed in copending applications Ser. Nos. 07/528,059, filed May 23, 1990, 07/598,984 filed Oct. 17, 1990, and Application No. 07/599,888 filed Oct. 18, 1990, all assigned to the same assignee as the present invention. The gas analyzers disclosed in the '858 and '859 patents and the copending applications are of the non-dispersive infrared radiation (NDIR) type. They operate on the premise that the concentration of a designated gas can be measured by: (1) passing a beam of electromagnetic radiation through the gas, and (2) then ascertaining the attenuated level of the energy in a narrow band absorbable by the designated gas. This is done with a detector capable of generating an electrical output proportional to the amount of energy absorbed by the gas.
One important application of the invention at the present time is in capnometers for monitoring the level of carbon dioxide in the breath of medical patients. This is typically done during a surgical procedure as an indication to the anesthesiologist of the patient's condition, for example. As the patient's wellbeing, and even his life, is at stake, it is of paramount importance that the carbon dioxide concentration be measured with great accuracy.
In a typical instrument or system employing non-dispersive infrared radiation to measure gas concentration, including those disclosed in the '858 and '859 patents and the copending applications, the electromagnetic radiation is emitted from a source and focused by a mirror on the gases being analyzed. After passing through the body of gases along an optical path, the beam of electromagnetic radiation passes through a filter. That filter reflects all of the radiation except for that in the narrow band centered on a frequency which is absorbed by the gas of concern. This narrow band of radiation is transmitted to a detector that is capable of producing an electrical output signal proportional in magnitude to the magnitude of the electromagnetic radiation impinging upon it. Thus, the radiation in the band passed by the filter is attenuated to an extent which is proportional to the concentration of the desired gas. The strength of the signal generated by the detector is consequentially inversely proportional to the concentration of the designated gas and can be inverted to provide a signal indicative of that concentration.
In a typical capnometer, the infrared radiation source and the detector are incorporated into a single transducer. This is assembled to an airway adapter, which is a device with a sampling passage for the gases being monitored.
Most NDIR gas analyzers use a ratioing scheme to eliminate errors attributable to drifts in the infrared radiation source and other parts of the systems and transmission losses. Three methods are common.
1. An optical chopper is used with a single detector. The chopper contains a reference cell or filter, and the detector signal alternates between that reference cell and the gas to be measured. A ratio is taken of these two signals.
2. Two detectors are located next to each other, and each is illuminated by one-half of the infrared radiation beam. A ratio is taken of the detector outputs. The reference channel is presumed to be responsive to any changes in the detected energy that are not due to the absorption of the designated gas, and the changes are presumed to be the same in both the reference and the data channels.
3. A beam splitter is placed in the optical path between a single infrared radiation source and data and reference detectors of like dimension. The radiation is passed through the gases being analyzed and divided by the beam splitter into moieties in which the energy is of wave lengths that are respectively shorter and longer than a designated wave length. The energy in these moieties is transmitted through appropriate band pass filters to the data and reference detectors. A ratio is taken of two detector outputs.
In the third of the foregoing schemes, the actual concentration CONC in *Torrs of a selected gas in the optical path is determined according to the following equations: EQU IX=SCV (ZCV-MR) (1) EQU CONC=Table (IX) (2)
In equation (1), IX is an index value, SCV is a Scan Cal Value, ZCV is a Zero Cal Value, and MR is the measured ratio of the data signal to the reference signal. The index value IX is a number used to cross-reference the Measured Ratio MR, after it is adjusted by the Zero and Span Cal Values, to a concentration table containing actual concentrations of a selected gas corresponding to different ratios of data signals to reference signals. The concentration table of Equation (2) is empirically generated by measuring the ratios of data signals to reference signals for different known concentrations of the gas of interest.
The Measured Ratio is the ratio of the signal through data path S.sub.D to the signal through the reference path S.sub.R for a given gas. It is given by the following equation: ##EQU1## where G.sub.D is the gain introduced through the data path, G.sub.R is the gain through the reference path, k is the absorption (extinction) coefficient of the designated gas at a specific wave length, l.sub.sc is the optical path length of a sample chamber containing the gas of interest, C.sub.m is the measured concentration of the selected gas, and L is the light leakage in the absorption band of the selected gas.
Zero Ratio (ZR) is the ratio of the data signal to the reference signal when the concentration of the gas being measured is zero. The Zero Ratio is given by the following equation: ##EQU2##
The Zero Ratio is measured by placing a sample in which the designated gas is absent in the optical path of the transducer unit and measuring the data and reference signals. While the zero ratio is being calculated, the voltages of the reference and data signals are set as close as possible to the same value using automatic gain control circuitry. The ratio of data to reference signals is thus ideally equal to unity.
For calibration purposes, a Span Ratio (SR) is also employed. The Span Ratio is the ratio of the data signal to the reference signal for a known concentration of selected gas Cs. Substituting the span concentration Cs into equation (3) yields the following equation: ##EQU3##
The Zero and Scan Cal Values are calculated from the Zero and Span Ratios according to the following equations: ##EQU4## where IX.sub.s is the index value corresponding to the known span concentration level C.sub.s.
In one prior method of calibration, known as gas flow calibration, an operator flows a sample in which the gas of interest is absent through an open chamber to obtain a zero ratio ZR. The operator then flows a mixture of the selected gas and another gas through the open chamber and obtains a span ratio SR. The percentage of selected gas in and flow volume of the sample is known; accordingly, the concentration of the sample gas of interest in the sample is known. The zero ratio ZR and span ratio SR thus obtained represent known concentration levels at two points and may be used to calculate Zero and Span Cal Factors according to equations (6) and (7) above.
In a second calibration method, referred to hereinafter as the gas cell method, the transducer analyzer is calibrated using known gas concentration levels of the selected or designated gas in two sealed cells. A first cell, known as the zero cell, does not contain the gas of interest. A second cell, known as the span cell, contains a known concentration of that gas. The operator measures the Zero Ratio by placing the zero cell in the optical path of the detector and the Span Ratio by placing the span cell in the optical path. As in the gas flow calibration method, once two points are known, the Zero and Span Cal Values may be calculated according to equations (6) and (7) above.
Many difficulties are manifest in these two calibration schemes. Both gas flow calibration and gas cell calibration require that two points be measured before the transducer can be calibrated. If the transducer is used to measure the concentration of CO.sub.2 exhaled by a patient during an operation, the time involved in measuring two points during calibration may be unacceptable.
Additionally, the following problems are specific to the gas flow calibration method.
It requires that two gas storage tanks be kept available for calibration. One gas storage tank is needed to supply the gas that is not the gas of interest in obtaining the zero ratio, and the other gas storage tank is required to supply the known mixture of the selected gas and the other gas used in obtaining the span ratio. Given the confined spaces of most operating rooms, it may be inconvenient or not feasible to keep two gas storage tanks on hand. The time required to set up a calibration procedure involving flowing two gases through the transducer calibration set-up may also be unacceptable.
Further, in the gas flow calibration method, the flow rate and percentage concentration of the known gas mixture must be carefully regulated to ensure that the actual concentration of the carbon dioxide or other selected gas in the known mixture closely corresponds to the span concentration level used to calculate calibration values. Should the flow rate vary from the desired value, inaccuracies in calculating gas concentrations may result.
The gas cell calibration method also has its own unique problems. The span cell containing the known concentration of the designated gas may leak, rendering the span ratio SR and calibration values calculated therefrom inaccurate. Further, construction of a sealed cell is difficult and expensive.